1. Field of the Invention
The present invention relates to an objective lens configured to focus multiple laser beams having different wavelengths on corresponding optical discs and also to an optical pickup apparatus including the objective lens.
2. Description of the Related Art
There are multiple standards for optical discs used as information recording media. The laser beam, the recording density, and the thickness of a coating layer for coating an information recording layer, which are used by the standards, are different among the standards. To put it specifically, the optical discs of the compact disc (CD) standard, the digital versatile disc (DVD) standard and the blue-ray disc (BD) standard are currently used.
In order to reduce the number of components, a single objective lens preferably supports the optical discs of these three standards. However, it has not been easy to develop an objective lens compatible in these three standards.
This is because conditions such as a wavelength of a laser beam are different among the standards. As an example, in the BD standard, the wavelength of a laser beam is 405 nm, the thickness of a covering layer that covers an information recording layer of an optical disc is 0.1 mm, and the numerical aperture of a used objective lens is 0.85. In the DVD standard, the wavelength of a laser beam is 650 nm, the thickness of a covering layer for an optical disc is 0.6 mm, and the numerical aperture of an objective lens is 0.60. In the CD standard, the wavelength of a laser beam is 780 nm, the thickness of a covering layer for an optical disc is 1.2 mm, and the numerical aperture of an objective lens is 0.45.
Optical super resolution is one technique of miniaturizing a focus spot without increasing the numerical aperture of the objective lens. An optical pickup apparatus including an objective lens employing the optical super resolution is described in Japanese Patent Application Publications Nos. Hei 9-69237 and 2003-45064, for example.
Japanese Patent Application Publication No. Hei 9-69237 discloses an invention in which a laser beam is partially attenuated to achieve the optical super resolution. Specifically, with reference to FIGS. 1 and 2 in this document, an attenuator 3 is provided in the middle of an optical path of the laser beam, and is used to partially attenuate a laser beam. Thus, a portion of the laser beam incident on a center portion of the objective lens 6 is attenuated and the optical super resolution is achieved.
Japanese Patent Application Publication No. 2003-45064 describes a structure in which the optical super resolution is achieved by the objective lens alone. Specifically, with reference to FIG. 2 in this document, an objective lens 8 includes a first region 8-1 in a center portion of the objective lens 8 and a second region 8-2 outside the first region 8-1, and diffuses a portion of a light beam incident on the first region 8-1. Thus, the portion of the light beam incident on the first region 8-1 is prevented from contributing to spot formation and the optical super resolution is achieved.